1. Field of the Invention
The present invention relates generally to integrated circuits having a self-contained voltage control oscillation circuit, and more particularly to such an integrated circuit having a self-contained voltage control oscillation circuit which outputs a plurality of oscillating signals for use in a signal processing circuit for audio, imaging equipment and the like.
2. Discussion of the Related Art
FIG. 4 shows an example of a conventional voltage control oscillation circuit. In FIG. 4, numeral reference 31 denotes a charge-discharge switching circuit, 32 a capacitor, 33 a discharge resistor and 34 a control voltage input terminal.
The charge-discharge switching circuit 31 is arranged to charge and discharge the capacitor 32 alternately, and the charging voltage has two threshold values: an upper and a lower threshold value. When a charging voltage reaches the upper threshold value in the capacitor 32, the charging of the capacitor 32 is stopped. Then the capacitor 32 is discharged by the discharge resistor 33 connected thereto in parallel and the charging voltage drops. When the charging voltage drops to the lower threshold value, the charge-discharge switching circuit 31 charges the capacitor 32.
Since the capacitor 32 is charged with current surpassing the discharging current due to the discharge resistor 33, the charging voltage rises and reaches the upper threshold value in the capacitor 32. The process mentioned above is repeated in that way and a saw tooth or triangular oscillating signal is output.
Further, either the upper or the lower threshold value varies according to the control voltage received via the control voltage input terminal 34, thus causing the amplitude of the oscillating signal to vary. On the other hand, charge-discharge constants for defining the inclination of the oscillating signal are fixed. Therefore, the oscillating frequency varies with the repetitive period. The oscillating frequency of the output signal is thus controlled by the control voltage.
However, an oscillating circuit of the sort set forth above needs an externally-installed capacitor with a relatively large capacity, and this tends to render such an oscillating circuit liable to frequency fluctuation due to aging change of the capacitor, thus necessitating arranging a regulating circuit (not shown).
FIG. 5 illustrates an exemplary voltage control oscillation circuit using a ceramic or quartz oscillator 41 in order to improve aging and temperature change characteristics. In this case, the oscillating frequency of the output signal is still controllable as the charge-discharge time constants of the capacitors 42 and 43 vary with the control voltage via a terminal 44.
FIG. 6 illustrates an exemplary oscillation circuit using an operational amplifier, wherein oscillation is made by charging and discharging a capacitor 52 via a resistor 51, and a capacitor 54 via a resistor 53. The resistance of this circuit is replaced by that of a circuit equivalent to a variable resistor, though the illustration thereof has been omitted. Moreover, the oscillating frequency is controlled as the apparent value of resistivity of the equivalent circuit varies with the control voltage.
In the case of the conventional integrated circuit having a self-contained voltage control oscillation circuit, the voltage control oscillation circuit has been incorporated in an integrated circuit. Since equipment of this sort is normally required to issue a plurality of oscillating signals, a plurality of oscillation circuits are integrated onto one chip, whereby the plurality of oscillating signals are supplied from one integrated circuit.
The circuits shown in FIGS. 4 and 6 need large capacity capacitors in order to obtain a precise oscillating frequency and a regulating circuit to offset fluctuations originating from variations in the capacity of capacitors.
The reason stated above thus makes impossible any attempt to form into an integrated circuit (IC) with the whole of such a conventional circuit having a plurality of self-contained voltage control oscillation circuits.
Although the oscillating frequency is easily stabilized with the circuit shown in FIG. 5, an output signal frequency directly available becomes considerably high as the oscillating frequency is determined by the natural oscillation inherent in a ceramic oscillator or the like.
Consequently, the frequency will have to be divided a number of times until a desired one is obtained and an area to be occupied by a frequency divider tends to increase sharply in the chip containing the integrated circuit. An increase in the chip area will result in decreasing not only a yield of chips but also a yield rate per wafer, and this is disadvantageous as it pushes up the IC production cost.
Moreover, the oscillator is relatively expensive and will have to be externally-installed. When it is attempted to obtain an output signal having an oscillating frequency unobtainable by dividing the frequency, each oscillation circuit also needs an externally-installed regulating circuit.
Thus externally-installed parts and regulating circuits in addition to such an integrated circuit having a self-contained voltage control oscillation circuit are required, and this is also disadvantageous as the packaging density lowers. Particularly, in the case of an integrated circuit having a plurality of self-contained voltage control oscillation circuits, each oscillation circuit needs regulating and the oscillating frequency characteristics of the output signals can hardly be uniformized. The problem is that a great deal of manhours required for regulating decreases production efficiency.